Sound, after all, is the number one signature “that gives away the location of low-altitude UAVs and gives away their presence.” Which sort of defeats the point of having a secret surveillance eye in the sky. In some cases, you might want people to know you’re watching. At other times, you want to sneak up quietly.

But it’s hard to do without sacrificing payload. The added weight of sensors, and the ability to operate for longer periods, comes with trading out stealthiness. Drones powered by batteries: They’re quiet, but can’t stay in the air for long. Then there’s the added noise caused by airflow generated from propellers, and noise from gasoline or diesel engines (not counting batteries), with their moving pistons, turbofan and gears.